Your elementary classes sound like my middle school exploratories. I
have a bazillion students and it's pass/fail so actual numeric grades
are kind of impossible and not really that important. What I do to
keep track is pretty cool, and it's gotten lots of smiles from
administrators, parents, and students alike. Since I teach mostly
multicultural art, I have the students make passports with the
countries already listed in them that we will be "visiting." Whenever
they complete their project for that country satisfactorily, I stamp
that area of their passport with a fun and appropriate stamp, and that
counts as "pass." This also helps when students are absent and need
to know what to make up. They don't even have to ask, they just check
their passport, see what the project was for that country, and make
one. (I also have the handouts and supplies arranged by country so
they can get their own.) The kids love the passports, and keep them
as keepsakes later on.

For my high schoolers, whose grades are more cut and dry, I use
rubrics and grade the projects as they finish them. I make my rubrics
at Rubistar: http://rubistar.4teachers.org/index.php
This has been a Godsend for me because I'm not that organized, so
piles and piles of projects usually means things get lost, graded
late, etc. If I grade projects while kids are finishing them, they
can just take them home and I have a clean desk. Plus, everything the
kid needs to do to get an A is right there on the rubric, so there's
no arguing over grades (if you share the rubric with them, first.)
There's also none of the "how come he got and A and I got a B" stuff,
either!